Fiber optic systems are used in many applications. One of the types of applications can be described as fiber optic sensor interrogators. In a typical fiber optic sensor interrogator, light is emitted from an interrogation unit containing a laser and other optical devices. The laser may be a continuous wave (CW) laser, it may be a pulsed laser, which may include a separate amplifier and pulse generator. Or it may be a naturally pulsed laser (for example a Nd:YAG laser) without need of separate amplification or pulsing circuitry. In addition, in a typical sensing application the interrogation system may contain an optical receiver to receive back-scattered signals from the sensor in order to make a measurement. In many applications, light that is emitted from the interrogator will reflect off of a sensor and return to the interrogator, for example a Fabry-Perot cavity, or fiber Bragg grating. Another method of sensing is to use the intrinsic backscattering of the fiber through scattering processes including Rayleigh, Brillouin, and Raman scattering. The scattering processes will provide a return signal back to the interrogator that is received at the detector to make a measurement of parameters like strain, vibration, and temperature.
An important design consideration in many fiber optic sensor interrogators is in applications in which the light from the interrogation unit passes into regions that that may contain explosive atmospheres, such as the subsurface environments of oil and gas wells.
Achieving intrinsic safety with any complex electrical device is very difficult because it requires that the available electrical energy at the device be limited below the level required for ignition. This requires that only low voltages and currents are used and that no significant energy storage can occur within the device. With a fiber optic sensor, the interrogator may be placed many tens or hundreds of meters away from the hazardous region with only the fiber optic cable and passive optical sensor being within the explosive atmosphere. For years it was thought that the energy present in fiber optic sensing systems was not high enough to cause ignition and additionally, all energy was contained inside the glass fiber, therefore it was safe to use in explosive atmospheres. However, in recent years, tests have been performed that demonstrate that in explosive atmospheres ideal for ignition, it is possible for a relatively low-power optical signal, on the order of 10s or 100s of milliwatts average power, to cause ignition. In the case of a broken fiber, optical power can exit the fiber and be absorbed by a small dust particle. The dust particle may absorb most of the optical power and due to its low surface area, heat can accumulate in the particle rapidly until the particle reaches a high enough temperature to cause ignition.
The optical power required for ignition depends on many factors including: core size of the fiber and beam diameter, pulse duration if pulsed light, wavelength of the light, components of the flammable gas mixture, and the presence of target particles. A number of experiments have been performed to determine a safe power threshold, below which ignition cannot occur even with the most explosive gas mixtures. A power level of 35 mW has been accepted as a safe threshold level, below which ignition due to optical radiation cannot occur.
These ignition power levels are not a concern for most fiber optic sensing systems when they are operating with normal power levels required for sensing. However, the capability exists within many of some interrogator designs to generate much higher power if a fault were to occur in the system. For example, a distributed sensing method like Distributed Temperature Sensing (DTS), Distributed Acoustic Sensing (DAS), etc. may interrogate a fiber optic sensing cable using an optical time domain reflectometry method whereby a short pulse of light, on the order of tens of nanoseconds or less, is sent into the fiber repeatedly at up to tens of kilohertz repetition rate. Typically, an electrical control circuit is used to generate the timing pulse, which is sent to an optical component that controls the timing and duration of the optical pulse. If a malfunction were to occur in this pulse generating circuit due to an electronics fault, or a fault in software/firmware that may be controlling the electronics, it will be possible for the optical pulse length to exceed the desired duration. In extreme cases, the pulse duration may grow to 10s or 100s or 1000s of times the normal duration, which will have the effect of increasing the average optical power by a proportional amount and may exceed the safe optical power level for operating in explosive atmospheres. Another possible fault may occur in any optical amplification component, for example an erbium-doped fiber amplifier (EDFA). The EDFA is given a control signal to set the gain to a desired level that is normally below the maximum gain that the EDFA is capable of generating. A fault in the electronics, firmware, or software controlling the EDFA may allow the gain level to exceed the desired level, allowing optical power levels to be emitted that are much higher than desired and may exceed the safety threshold for explosive atmospheres.
Prior art methods of power regulation, for example in fiber optic telecom systems, have been to use a device to monitor the power of the transmitted light by using a circulator/coupler to redirect a small percentage of the light to an optical detector. When the power indicated by the optical detector increases beyond a threshold value, an optical switch or variable optical attenuator is adjusted to attenuate the outgoing light. An electronic control circuit is used to coordinate these components. A disadvantage of such approaches though is that they involve active devices that have their own failure modes. If any one of these three components were to fail to operate properly, the safety mechanism may fail to operate.
There is a need then to move beyond these active systems to find in fiber optic interrogator systems that are more fail safe.